50% Grey at 0.5 instead of 0.18?
The color wheel has three options. By default RGB, HSV and Hex(adecimal). The first option, RGB, uses linear color, but the second and the third are color managed. By default they uses the sRGB color space in you display device.

This question has been noticed by many blender users. You can see further details in this blender development task.

If you want to use linear color in all color wheel options, you need to modify the default set up for color management, and set None in the Display Device of you BLEND file.